1. Field of the Invention
The present invention relates, in general, to heat pump apparatuses and, more particularly, to a compression-ratio control device for such heat pump apparatuses.
2. Description of the Prior Art
A heat pump apparatus is an air conditioning machine that is operated with a reversed Carnot cycle to be selectively used as a heater or a cooler. As shown in FIG. 1, the basic refrigeration circuit 1 of the cycle comprises a compressor 2, a four-way valve 3, an indoor heat exchanger 4, a cooling-mode expansion valve 5, a heating-mode expansion valve 6 and an outdoor heat exchanger 7 which are sequentially connected to each other through a main refrigerant line, consisting of a plurality of refrigerant pipes 8a, 8b, 8c and 8d, such that the first pipe 8a starts at the outlet port of the compressor 2 and the fourth pipe 8d is ended at the four-way valve 3. In addition, the four-way valve 3 is also connected to the inlet port of the compressor 2 through a refrigerant suction line 8e. 
In order to perform a heating-mode operation of the heat pump apparatus, the four-way valve 3 is controlled such that refrigerant flows through the refrigeration circuit in a direction as shown by the solid arrows of the drawing. During such a heating-mode operation, high pressure and high temperature gas refrigerant outputted from the compressor 2 flows to the indoor heat exchanger 4, which acts as a condenser for condensing the gas refrigerant while transmitting heat from the gas refrigerant to indoor air or water flowing around the indoor heat exchanger 4, thus heating the indoor air, producing hot water or accomplishing a drying function while condensing the gas refrigerant. The high pressure and high temperature liquid refrigerant outputted from the indoor heat exchanger 4 is expanded in the heating-mode expansion valve 6. The refrigerant from the expansion valve 6 is, thereafter, evaporated by heat at the outdoor heat exchanger 7 acting as an evaporator using a surrounding fluid, such as outdoor air used as a heat source. At the outdoor heat exchanger 7, the liquid refrigerant thus becomes low pressure and low temperature gas refrigerant, which is returned to the compressor 2 through the refrigerant suction line 8e so as to accomplish one cycle. During the heating-mode operation, the heat pump apparatus repeats the above-mentioned cycle.
When it is required to perform a cooling-mode operation of the heat pump apparatus, the four-way valve 3 is controlled such that refrigerant flows through the refrigeration circuit in a direction as shown by the dotted arrows of the drawing. During such a cooling-mode operation, high pressure and high temperature gas refrigerant outputted from the compressor 2 flows to the outdoor heat exchanger 7, which acts as a condenser for condensing the gas refrigerant. High pressure and high temperature liquid refrigerant outputted from the outdoor heat exchanger 7 flows to the cooling-mode expansion valve 5, thus being expanded in the valve 5. Liquid refrigerant from the cooling-mode expansion valve 5 is fed to the indoor heat exchanger 4 acting as an evaporator, and evaporated by heat transmitted from indoor air or water flowing around the indoor heat exchanger 4. Due to the evaporation of the refrigerant at the indoor heat exchanger 4, the refrigerant cools the indoor air or produces cold water. Thereafter, low pressure and low temperature gas refrigerant is returned from the indoor heat exchanger 4 to the compressor 2 so as to accomplish one cycle. During the cooling-mode operation, the heat pump apparatus repeats the above-mentioned cycle.
During a heating-mode operation of the conventional heat pump apparatus, the coefficient of performance (COP) of the apparatus is increased in proportion to the quantity of heat transmitted from the refrigerant to the indoor air or water at the indoor heat exchanger 4 acting as a condenser. The COP of the heat pump apparatus during a cooling-mode operation is increased in proportion to the quantity of heat transmitted from the indoor air or water to the refrigerant at the indoor heat exchanger 4 acting as an evaporator. When the quantity of heat, transmitted from the refrigerant to the indoor air or water at the indoor heat exchanger 4 acting as a condenser during a heating-mode operation, is increased in an effort to increase the COP of the apparatus, the temperature of the output gas refrigerant from the compressor 2 rises. In addition, when the temperature of outdoor air during such a heating-mode operation is lowered, the quantity of heat absorbed by the refrigerant at the outdoor heat exchanger 7 acting as an evaporator is reduced in proportion to a reduction in the temperature of the outdoor air. During a heating-mode operation of the conventional heat pump apparatus in the case of either of the above-mentioned two cases, the compression ratio of the compressor 2 is increased. During a cooling-mode operation of the heat pump apparatus at a high temperature of outdoor air, gas refrigerant may be incompletely condensed at the outdoor heat exchanger 7 acting as a condenser. In such a case, the difference between the condensing temperature and the evaporating temperature is enlarged, so the compression ratio of the compressor 2 is increased.
In the case of such an increase in the compression ratio of the compressor 2, the temperature of output gas refrigerant from the compressor 2 is increased, thus sometimes overheating the compressor and thermally degrading lubrication oil to lower the operational reliability of the compressor, as well as reducing volumetric and compression efficiencies of the compressor. This results in a reduction in the COP of the apparatus. In an effort to solve the problems, the compressor of a conventional heat pump apparatus may be provided with a high voltage protective switch or an inverter compressor may be used as the compressor of the heat pump apparatus, thus accomplishing a low rpm of the compressor as well as controlling the compression ratio of the compressor.
Such a conventional method may somewhat effectively control the compression ratio of compressors and does not reduce the COP of the heat pump apparatuses in the case of a heating-mode operation at an outdoor temperature not lower than 5xc2x0 C. However, the method is problematic in that it is almost impossible to completely defrost the outdoor heat exchanger 7 in the case of a heating-mode operation at a cold outdoor temperature lower than 5xc2x0 C., even though a defrosting means installed around the heat exchanger 7 is operated. Particularly in the coldest season, the quantity of frost formed on the outdoor heat exchanger 7 is excessively increased, so the evaporation efficiency of liquid refrigerant at the outdoor heat exchanger 7 is reduced. Such a reduction in the refrigerant evaporation efficiency at the outdoor heat exchanger 7 sometimes results in disabling of the heat pump apparatus.
Accordingly, the present invention has been made keeping in mind the above problems occurring in the prior art, and an object of the present invention is to provide a heat pump apparatus, which improves the operational reliability of its compressor and maintains its COP at a desired level.
In order to accomplish the above objects, the present invention provides a heat pump apparatus, comprising a basic refrigeration circuit including a main refrigerant line consisting of a first refrigerant pipe extending from a compressor to a four-way valve, second and third refrigerant pipes sequentially connecting the four-way valve, an indoor heat exchanger, a cooling-mode expansion valve, a heating-mode expansion valve and an outdoor heat exchanger to each other, and a fourth refrigerant pipe extending from the outdoor heat exchanger to the four-way valve; and a refrigerant suction line extending from the four-way valve to the compressor, further comprising: a main bypass line extending from the main refrigerant line at a position between the cooling-mode expansion valve and the heating-mode expansion valve installed on the third refrigerant pipe to the refrigerant suction line, with a liquid refrigerant tank installed on the main bypass line; a pressure control valve and a solenoid valve installed on the main bypass line at positions around the inlet and outlet ports of the liquid refrigerant tank, respectively; a second bypass line extending from the second refrigerant pipe at a position between the four-way valve and the indoor heat exchanger to the fourth refrigerant pipe; and a plurality of capillary tubes installed in the liquid refrigerant tank and connected to both the second bypass line and a branch line branching from the main bypass line at a position around the inlet port of the pressure control valve.